A configuration of the path of a laser beam and an optical element is disclosed in U.S. Pat. No. 5,825,562. In the configuration of U.S. Pat. No. 5,825,562, the lifetime of an optical element under laser irradiation is prolonged in that the optical element is gradually or continuously moved further in the laser beam to always use new regions of the optical element. The optical element is disposed such that it can be displaced in two directions perpendicular to each other, and is moved on a circular trajectory whose center simultaneously follows a spiral path. This configuration, however, requires a lot of construction effort and is susceptible to disturbances due to the large number of movable components.